Add Pre-Con and Post-Con tabs to the schedule grid for early/late events

[dkapell]

If a convention has events scheduled (manually, via sql query) before the start of the event, they should be listed in a tab before the first day of the event on the schedule grid, but the schedule grid view should default to the first actual day of the event. same with events after the end.
Use case: pre-intercon meet-and-greets, post-intercon community meeting

[marleighnorton]

Looked this over and yeah, just adding Pre-Con and Post-Con tabs that are always in List View seems like a fine plan.